Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, which is incorporated herein by reference in their entirety. The carbonylation catalyst contains a metal catalyst, such as rhodium, which is either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
Methanol and carbon monoxide are fed to a reactor as feedstocks. A portion of the reaction medium is continuously withdrawn and provided to a flasher where the product is flashed and sent as a vapor to a purification train. The purification train includes a light ends column which removes “light” or low boiling components as an overhead and provides a side stream for further purification. The purification train may further include columns to dehydrate the side stream or for removing “heavy” or high boiling components, such as propionic acid, from the side stream. It is desirable in a carbonylation process for making acetic acid to minimize the number of distillation operations to minimize energy usage in the process. U.S. Pat. No. 5,416,237 discloses a process for the production of acetic acid by carbonylation of methanol in the presence of a rhodium catalyst, methyl iodide, and an iodide salt stabilizer. The improvement according to the '237 patent resides in maintaining a finite concentration of water up to about 10 wt. % and a methyl acetate concentration of at least 2 wt. % in the liquid reaction composition and recovering the acetic acid product by passing the liquid reaction composition through a flash zone to produce a vapor fraction which is passed to a single distillation column from which the acetic acid product is removed. The drawback of eliminating distillation stages is that the level of purity of the product suffers. In particular, the distillation columns tend to remove high boiling iodides as well as aldehyde contamination products. Both of these impurities impact the commercial desirability of the final product.
U.S. Pat. No. 6,657,078 discloses a low energy process for producing acetic acid by the carbonylation of methanol. The process involves a rhodium-catalyzed system operated at less than about 14 wt. % water utilizing up to 2 distillation columns.
US Pub. No. 2013/0116470 discloses a production process of acetic acid comprises a reaction step for continuously allowing at least one member selected from the group consisting of methanol, dimethyl ether, and methyl acetate to react with carbon monoxide in a catalyst system comprising a rhodium catalyst, an iodide salt, and methyl iodide in the presence of acetic acid and water in a plant compromising a reactor, a flasher, and a distillation column, and wherein part of the vaporized stream is introduced into a heat exchanger. The liquid portion that is condensed from the vaporized stream is returned to the reactor for cooling purposes. As a result of a need to cool the reactor, the process disclosed in US Pub. No. 2013/0116470 increases the recycle of acetic acid which reduces carbon monoxide efficiency.
A frequent production limitation in the purification section of an acetic acid unit is the light ends column. In a typical acetic acid methanol carbonylation process, hot high pressure liquid from the reactor is reduced in pressure across a valve and flashed in a lower pressure flasher vessel. The vapors liberated from this step are fed near the bottom of a light ends column. Condensed liquids rich in acetic acid are removed from a liquid sidedraw above the feed and fed forward for further purification, while vapors exiting the tower overhead are condensed and fed to a liquid-liquid decanter. The light phase from the decanter is refluxed to the tower and the heavy phase is recycled to the reactor feed. Thus, the light ends column receives a hot vapor product stream from the flasher and operates to remove most of the methyl acetate and methyl iodide from the stream before the product stream is fed forward for water removal. U.S. Pat. No. 8,173,076 describes an improved apparatus and method of producing acetic acid which includes condensing overhead vapor to provide reflux to the light ends column as well as condensing vapor from a central portion of the light ends column to increase capacity.
Thus, what is needed is an improvement to an acetic acid production process that does not result in increased recycle of acetic acid and improves the operation of the light ends column.